Device for determining the dielectric constant of materials. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The device can be useful in amateur radio practice in assessing the dielectric constant of samples of plastics, ceramics, and other insulating materials, as well as for specialists and collectors in identifying and systematizing mineral samples. With a variety of capacitive sensor designs, it is possible to significantly expand the capabilities of the device.

The device is designed to determine the dielectric constant of plastics, minerals and ceramics and identify them by this parameter. The idea of ​​creating the device and the development of the sensor belongs to Ph.D. chem. Sciences G. G. Petrzhik. The device can be used by radio amateurs and professionals involved in the collection, collection and processing of minerals. The principle of determining the dielectric constant is based on an increase in the capacitance of the sensor when its surface is in close contact with the ground surface of the dielectric (mineral) and a corresponding increase in the transmission coefficient of the high-frequency signal in the measuring circuit with this capacitive sensor.

On fig. 1 shows the electrical circuit of the device.

Rice. 1 (click to enlarge)

On the transistor VT1, inductor L2, capacitors C1-C3 and resistors R1-R3, a harmonic oscillation generator with a frequency of about 2,5 MHz is assembled. From the output of the generator, the signal is fed to one electrode of the comb structure of the capacitive sensor B1. From another similar electrode, the signal induced through the capacitance of the sensor is fed to a detector made on the diode VD1 and an integrating RC circuit R10C9. This detector has a relatively low input impedance and is therefore less susceptible to RF interference and interference. The inductor L3 also serves to minimize interference from the network to the sensor, which represents low resistance for low frequencies. The rectified voltage at the input of the analog-to-digital converter is almost proportional to the permittivity of the sensor substrate and the material sample located on the sensor. An ADC with a 3,5-digit LCD digital display (HG1) acts as a millivoltmeter. The inverter on the transistor VT2 creates the signal necessary to highlight the point between the second and third characters of the indicator. The maximum value of the dielectric constant shown by the indicator is 19,99.

The power supply of the device is autonomous from the "Korund" battery or a 9 V battery (for example, "Nika", 7D-0125D).

On fig. Figure 2 shows a sketch of the design of a dielectric meter with a capacitive sensor, which is located outside the plastic case with dimensions of 80x70x35 mm, used by the author from the antenna amplifier (TAU-1). The second version of the design differs from that shown in Fig. 2 in that the sensor is located on the side opposite to the indicator. In this case, it is convenient to place the device on top of a large array of the identified mineral.

Fig. 2

Inside the body of the device there is a battery and a printed circuit board with the rest of the device elements - on one side of the board, and an LCD indicator - on the other. Rectangular holes of corresponding sizes are cut out in the housing for the indicator and the sensor. The holes for adjusting the trimmer resistors must be accessible and located so that during calibration they do not interfere with the position of the sample on the surface of the sensor and the observation of the readings.

The plate of the capacitive sensor V1 is made of one-sided foiled fiberglass with plates etched or cut out of metallization with a width of conductors and gaps between them of 0,8 ... 1 mm with a width of "combs" of 8 ... 10 mm. The sensor is attached to the body with countersunk screws M2,5 on insulating sleeves 8...10 mm high. Other options for mounting the sensor are also possible. Inside the housing between the sensor and the electronic unit at a distance of at least 10 mm, an electric shield made of bronze or copper foil should be placed to reduce the influence of hands on the readings during calibration and measurement. The wires connecting the sensor to the device and the screw heads must not protrude above the combs. The sample of the studied material superimposed on the sensor should cover the entire surface of the "comb".

The oscillatory circuit of the generator is made on the basis of the DPM-0,1 (L2) choke and capacitors C2, C3. The communication coil L1 has 20 turns of PELSHO 0,15 wire wound over the choke coil. The same inductor is used as inductor L3.

Capacitors C1-C3, C7, C9, C11, C12 - mica, ceramic thermostable TKE groups (i.e., except for H10-H90) or K73 film groups; C5, C8 are also ceramic.

Instead of the D9E diode, you can use another germanium one - for example, D18, GD503A.

Before starting measurements, the device must be calibrated, for which, by turning on the power, using the tuned resistors R4, R7, brought into the holes in the housing for adjustment under the slot, the indicator readings are obtained corresponding to the relative permittivity of air er = 1 and a material sample with a known value of the parameter er. The DC voltage at the detector output must be within the limits sufficient to set the indicator readings in three digits - 4 with a trimming resistor R1,00. Then, having tightly applied to the sensor a smooth (polished) surface of a material sample with a known dielectric constant, which has a small spread (for example, getinax - its er = 5), set the readings of the LCD indicator using the trimmer resistor R7 in accordance with the value of the dielectric constant of the selected calibration material. By repeating the calibration by adjusting the resistor R4, the readings are clarified, corresponding to the values ​​of the dielectric constant of the air and the sample used. The surfaces of the materials to be identified, having a contact area smaller than the dimensions of the sensor, must be the same in thickness and area with the sample used for calibration. In other conditions and tasks, the sensor may have a different design, due to the shape, size and physical state of the samples.

Polystyrene, plexiglass, marble can also be recommended as calibration sample materials (the table shows the values ​​of the relative permittivity of solid dielectric materials used, in particular, in radio engineering and electronics). For the specified dimensions of the capacitive sensor, the thickness of the dielectric under study must be at least 5 mm, otherwise the real value of the parameter will be underestimated.

The device actually conducts relative measurements, comparing the dielectric properties of a known dielectric and a sample of the material under study. The closer they are in terms of the value of the estimated parameter, the smaller the error in the measurement of the parameter; similar sizes and drying of the samples also help to improve the accuracy of the readings.

Author: L. Kompanenko, Moscow

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